1. Field of the invention
This invention relates to the prevention of wax deposits from wax-containing oil, and more particularly to a method for inhibiting the formation of wax deposits in oil wells producing said wax-containing oils.
2. Description of the Prior Art
In the production of most wax-containing crude petroleum drawn from subterranean formations a major problem occurs when the oil is cooled below the solidification temperature of the wax therein. When this happens, the wax solidifies and tends to precipitate out and deposit on the piping within the well, as well as any other equipment which may subsequently be contacted by the cooled petroleum. These precipitates are mostly straight-chain paraffin hydrocarbons having empirical structures ranging from C.sub.18 H.sub.38 to C.sub.38 H.sub.78.
While the solid precipitates can be crystalline or amorphous, it is generally believed that, in most cases, paraffins precipitate from crude oil as crystalline dendrites having branches radiating out from a central spine. Because of these branches, the crystals easily agglomerate to form interlocking networks which absorb oil and form thick, voluminous gel-like structures which restrict and, in extreme cases, halt the flow of the oil from the well. Even a small amount of wax can significantly lower the amount of oil which can easily be removed from the well. It is known that wax tends to deposit more readily on rough surfaces than on a smooth surface so that the rough metal surfaces typically found on the interior of well piping are ideally suited for such deposition. Once deposition starts, such deposition usually continues unabated until the well is plugged and oil production stopped, and costly and often frequent cleaning are then required to maintain operation.
Various techniques for removing such deposits from wells have been proposed. For example, mechanical scrappers have been employed to maintain the interior surfaces of the well conduits free of wax accumulations. Another method involves pumping an oil, heated to a temperature which is above the melting point of the wax, back into the well. This will melt and dissolve the wax, thus freeing the well string and for a time, at least, bringing production back up to normal. However, both of these approaches are costly, complex, and only function to remove already formed wax accumulations rather than prevent the wax accumulation from forming initially.
Still another approach to this problem is the application of one or more polymeric "wax inhibitors" to prevent the wax deposits from forming in the first place. The exact mechanism by which these materials inhibit wax deposition and accumulation is not fully understood. In use, it is found that, in most cases, the inhibitor does not seem to reduce the size or change the general appearance of the individual wax crystals. Rather, it seems to change the surface characteristics of the crystals so that they exhibit a reduced tendency to adhere either to each other or to solid surfaces and, thus, are more readily kept in suspension. These inhibitors therefore differ from conventional pour-point depressants which generally reduce the low temperature viscosity of many types of viscous fluids, without necessarily affecting their wax deposition characteristics.
Although a number of copolymers have been developed for this purpose, most of these are applied to the petroleum after it has been brought to the surface to prevent the wax from fouling pumps and other equipment involved in the transportation and processing of the oil. However, most of these materials are expensive and/or must be used in concentrations in the range of about 1000 to 10,000 ppm to be effective.
The effectiveness of a wax-inhibitor in a given situation is highly unpredictable. For example, it has been found that even closely related polymers, composed of the same monomers but of somewhat different monomeric contents and/or average molecular weight, can exhibit radically different wax-inhibiting properties, both above ground and, most particularly, in the more hostile environmental conditions typically found in subterranean hot oil pools and in well piping leading therefrom to the surface. Likewise, merely because a particular wax inhibitor proves useful in one environment provides no indication as to its effectiveness in another e.g., an inhibitor highly effective for a waxy distillate may prove substantially less effective, or even ineffective, for use in the oil pools of subterranean formations, and vice versa. As a result, finding a superlative inhibitor for down-hole use, even within a class of polymers known to exhibit some degree of wax inhibition, is a task considered extremely difficult due to the inherent unpredictability involved.